Many portable electronic devices are powered by rechargeable batteries where use of primary (non-rechargeable) battery technology would be prohibitively expensive given the rate at which the battery charge is depleted by the device. In some devices it is important to monitor the battery charge level as the battery is discharged to avoid undesired operation due to low voltage. Typically, when a battery reaches an end of charge condition and becomes fully discharged, the battery voltage begins to fall rapidly. Accordingly, it is common to set or select a shutdown threshold in the device where, upon the battery voltage falling to the shutdown threshold, the device will initiate a shutdown process to shut off the device and avoid undesired operation. The shutdown threshold is selected to give sufficient time to shut down device circuitry on the assumption that the battery is discharged or nearly discharged (and as a result, the voltage is dropping rapidly). The shutdown threshold is typically selected without regard for the amount of current being sourced by the battery to the device.
As an example of circuitry that can necessitate a shutdown, devices operating at low battery voltages (i.e., singe cell 3.6V) often require a DC converter to boost the battery voltage up to a stable voltage that is higher than the battery voltage. These DC/DC regulators require a minimum input voltage to produce the desired output boosted voltage, and may be either always on or activated as needed while the battery voltage decreases as it discharges during use. Either boost or buck/boost regulators may be used, depending on the application, and both provide stable and predictable power output at a predetermined constant voltage.
Typically battery voltage monitoring involves sampling the voltage at, or as close as physically possible to, the positive battery terminal where the device and battery connect. When the battery voltage drops below the set or selected shutdown threshold, a power management circuit can initiate steps to shut down the device. The shutdown threshold can be either fixed or programmable, and it is often defined by different limitations such as battery capacity or the presence of circuits, which are powered directly by the battery, shutting off
A problem can arise when a device draw a high peak current from the battery (this inrush might be due to a radio transmit situation—for example, during radio transmission). The current drawn from the battery, combined with a finite, but significant amount of internal battery impedance, causes the sampled battery voltage to fall below the shutdown threshold used by the power management circuit. Accordingly, the power management circuit detects a low battery voltage condition and performs one or more series of actions that result in a power-down of the device (this is also often done to protect the cells from over-discharge).
However, a transient drop in battery voltage does not necessarily indicate a discharged or nearly discharged battery (where the cell voltage falls below some manufacturer guaranteed limit). For example, when a battery is very cold it can have an increased internal resistance that, during a high current demand, results in the battery voltage (as sampled at the device's positive battery contact) dropping below a shutdown threshold even when the battery has significant charge remaining, and thus, while under the shutdown threshold, the battery voltage will remain relatively steady, rather than drop quickly as occurs with a nearly discharged battery. As a result, it is possible to have a charged battery, but a have a battery voltage at a level that would “trick” the power management circuit into acting as if the battery is at an end of discharge condition, thus causing the shutdown procedure to commence.
Accordingly, there is a need for a method and apparatus for continuing device operation when the voltage falls below a shutdown threshold due to a high current condition.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.